DECATUR — A former Marine, already irritated about the disappearance of $100 from his bank account, tackled a suspected robber who came into the bank wielding a fire extinguisher and demanding cash.Timothy Armstead was waiting at the Washington Mutual Bank branch on Tuesday to discuss the missing money when the man came in and told bank employees he had a bomb. The man gave them five minutes to get $2,000 in $50 bills, DeKalb County police said.